Modified to allow transparent widget proxies.

.project

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>kivymod</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.python.pydev.PyDevBuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.python.pydev.pythonNature</nature>
+	</natures>
+</projectDescription>

.pydevproject

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<?eclipse-pydev version="1.0"?><pydev_project>
+<pydev_pathproperty name="org.python.pydev.PROJECT_SOURCE_PATH">
+<path>/kivymod</path>
+</pydev_pathproperty>
+<pydev_property name="org.python.pydev.PYTHON_PROJECT_VERSION">python 2.7</pydev_property>
+<pydev_property name="org.python.pydev.PYTHON_PROJECT_INTERPRETER">Default</pydev_property>
+</pydev_project>

kivy/uix/widget.py

@@ -88,6 +88,23 @@ from kivy.base import EventLoop
 from kivy.lang import Builder
 
 
+class IClass(object):
+    '''Used to register 'widget equivalent' types.  Emulates
+    isinstance(widget, Widget) with isinstance(widget, WidgetLike).  This provides
+    the ability to use transparent widget proxies.'''
+    def __init__(self):
+        self.implementors = set()
+    def register(self,C):
+        self.implementors.add(C)
+    def __instancecheck__(self,x):
+        return self.__subclasscheck__(type(x))
+    def __subclasscheck__(self,sub):
+        return any(c in self.implementors for c in sub.mro())
+
+#Instantiate the IClass registry so we can compare to WidgetLike
+WidgetLike = IClass()
+
+
 class WidgetException(Exception):
     '''Fired when the widget gets an exception.
     '''
@@ -263,7 +280,10 @@ class Widget(EventDispatcher):
         '''
         if widget is self:
             raise WidgetException('You cannot add yourself in a Widget')
-        if not isinstance(widget, Widget):
+        #Use WidgetLike for isinstance comparisons
+        global WidgetLike
+#        if not isinstance(widget, Widget):
+        if not isinstance(widget, WidgetLike):
             raise WidgetException(
                 'add_widget() can be used only with Widget classes.')
         parent = widget.parent
@@ -605,3 +625,5 @@ class Widget(EventDispatcher):
 
     See :class:`~kivy.graphics.Canvas` for more information about the usage.
     '''
+# Register Widget as a 'widget equivalent' type
+WidgetLike.register(Widget)